Si nanowires as sensors: choosing the right surface.

We show, using ab initio calculations based on density functional theory, that for hydrogen-passivated Si nanowires (SiNWs), the relative contribution of surface atoms to the band-edge states varies according to the way these surface atoms are bonded to the core ones. The largest influence occurs when these bonds are oriented along the wire's growth direction, which occurs either on the symmetric (001) 1 x 1 or the monohydrated (111) 1 x 1 surfaces. These results are obtained for wires grown along the [110] direction, with hexagonal cross-sections and facets corresponding to (111) and (001) planes, as observed experimentally. Their diameters range from about 10 to 35 A. On the basis of our results, we propose that particular facets should be more appropriate to be functionalized in order to build SiNW-based sensors. As an example, we have investigated the effect of NH2 adsorbed on some of these surfaces on the electronic structure of these wires.